Wafers of glass, crystals, gem stones and other materials are pervasively polished in industry to obtain highly reflective surfaces. In the solid state electronics industry, such wafers typically include group III, IV and V materials in thin, usually disc-like shapes. Such wafers have an active side which is highly polished to facilitate formation of devices therein.
Apparatus for polishing such wafers include a carrier plate to which wafers are adhered with exposed surfaces downward upon, and in forced engagement with, a polishing pad on a platen. The platen and carrier are typically rotated at different velocities and/or directions causing relative lateral motion between the wafer surfaces and the pad. A slurry containing abrasive compounds, chemicals and water is provided at the pad/wafer interface to aid in the polishing process.
Wafers have been mounted to carriers by many methods in the past with varying degrees of success. For example, mechanical means were utilized such as wax mounts, mechanical cups, pin restraints and vacuum devices which were costly and their use untidy and time consuming. Much effort has been expended to develop for carriers, mounting pads which would facilitate free mounting of wafers, i.e., pads which would develop enough adherency due to friction, liquid tension, suction or similar phenomena to hold wafers freely on a pad without mechanical restraints.
Although mechanical restraints are still utilized for some applications, a large portion of wafer polishing is now accomplished by free mounting, employing a composite pad. The pad typically includes a relatively firm outer layer and a compressible layer including a fiber matrix for cementing to a carrier. The outer layer provides a mounting surface which is wetted and sometimes treated with a chemical to promote adhesion. The wafers are thoroughly cleaned and their mounting surfaces are sometimes treated to promote adhesion. Consequently, the condition of pad surfaces and wafers combine with polishing forces to seat the wafers so firmly on a carrier that they are difficult to demount without breakage. Such breakage is particularly evident in demounting large wafers or polysilicon wafers having formed therein a pattern of monocrystalline sites surrounded by oxide layers.
In the known methods of demounting wafers, a carrier is removed from a polisher and inverted at a workbench or sink. Typically, a tool such as a knife blade or a tweezer is utilized at an edge to pry a wafer from a pad surface. Sometimes a vacuum pickup tool is applied to the exposed face of the wafer to assist in demounting. These and similar mechanical methods involve an ever present risk of injury, whereby a small percentage of wafers are broken or have edges badly chipped.
Other prior art methods of handling wafers include thermal manipulations to expand and contact mounting pads. For example, heat and pressure are sometimes utilized to seat wafers to pads and chilling is utilized to break the seals. In another demounting method, a carrier is inverted in a sink and a pulsating water jet is applied at the edge of each wafer until it is dislodged and removed. The thermal manipulation method is costly in time and equipment. The water jet method reduces breakage but has not been readily accepted because the demounting time varies depending upon the bond between an adherent surface and a wafer and back spray sometimes strikes clothing and eyeshields, causing annoyance to operators.
Accordingly, it is desirable to provide new and improved expedients for demounting wafers. Such demounting should be done at least as economically and expeditiously as was done in the prior art, but without previous injury to wafers and annoyance to operators. It is particularly desirable to demount wafers without removing carriers from polishing machines. Such demounting should be amenable to large wafers and polysilicon wafers having complex, fragile structures.